1. Field of the Invention
The present invention relates to a four-wheel steering system in automotive vehicles, and more particularly to an electric control apparatus for the four-wheel steering system for controlling an actual steering angle of the rear road wheels to a target steering angle calculated in accordance with conditions of sensors detecting control factors for the rear road wheels.
2. Description of the Prior Art
In such a conventional control apparatus for a rear wheel steering mechanism as disclosed in Japanese Patent Early Publication No. 1-212667, a hydro-mechanical tracer mechanism is adapted to steer a set of rear road wheels at a calculated target steering angle. The hydro-mechanical tracer mechanism includes a fluid control valve the valve element of which is drivingly connected to an electric step motor to be axially displaced in proportion to leftward or rightward rotation of the step motor for controlling the steering angle of the rear road wheels. In the hydromechanical tracer mechanism, however, the fluid control valve is conditioned insensitive when the valve element is being displaced in a slight distance from its neutral position. For this reason, the rear road wheels may not be steered from their neutral positions even if the valve element is displaced in a slight distance from its neutral position in proportion to rotation of the step motor. This causes a difference between the actual steering angle of the rear road wheels and the calculated target steering angle. As a result, the rear road wheels may not be steered at the calculated target steering angle, and the steering response of the rear road wheels is deteriorated due to a time delay at the neutral position.